In wireless communication applications, such as cellular phone services or other wireless services, amplifiers are used to provide the desired signal coverage for the particular wireless application. For example, radio frequency (RF) power amplifiers are used for boosting the level of an RF signal prior to transmission of that signal. RF power amplification techniques, and particularly RF power amplification techniques used for wireless applications, have inherent drawbacks to which the industry continues to direct its efforts. Specifically, in developing an RF transmission system, considerable attention is given to amplifier efficiency and signal distortion of the amplified signal.
Amplifier efficiency, which is generally defined as the level of RF power that may be achieved at the output signal compared to the power that is input into the overall amplification process, is conventionally somewhat low in wireless applications. Therefore, considerable attention within the power amplifier industry has been devoted to methods of enhancing power amplifier efficiency. Small increases in amplifier efficiency can provide significant benefits in a wireless system and reduce the overall costs necessary to run the system.
Another drawback in RF power amplification, which must be addressed and taken into account with any methods for improving efficiency, is signal distortion. An RF power amplifier, to a greater or lesser extent, exerts a distorting effect on the RF signals that are amplified. Non-linearities of the amplifier, as well as other factors, contribute to the distortion. Such distortion must be controlled to ensure that the RF transmitter meets the various standards regarding RF interference.
To address amplifier efficiency, one current technique involves the use of envelope tracking of the input signal to the amplifier and use of the detected envelope to vary the amplifier operation. In an envelope tracking system, a variable power supply is utilized for supplying power to the amplifier. The envelope power levels of the input signal are monitored, and the power that is supplied to the power amplifier, or typically to the final stage(s) of the power amplifier, is varied based on the monitored envelope levels. More specifically, the power that is supplied to the amplifier is varied so as to be just sufficient to reproduce the power level required by the amplifier at a given instant of time. Therefore, at low envelope power levels, a low supply voltage is provided to the amplifier, and the full supply voltage is provided to the amplifier only when the maximum power is required, that is, at the envelope peaks.
However, while envelope-tracking techniques improve efficiency, it is desirable to improve upon envelope tracking features. Particularly, it is desirable to improve upon and efficiently incorporate an envelope tracking power supply into a transceiver. Furthermore, it is desirable to improve upon the efficiency and linearity of an RF power amplifier, in a transceiver system. Still further, it is desirable to utilize the digital signal processing capabilities of a transceiver for implementing envelope tracking capabilities. It is further desirable to implement such features of envelope tracking while addressing imperfections or non-linearities in the tracking behavior of the power supply,